1. Field of the Invention:
This invention relates to structural articles prepared from structural fibers or stranded fibers which may be saturated with a curable resin, and thereafter incorporated into an article of manufacture as a structural element. More particularly, this invention relates to articles comprised of densely packed, parallel and oriented structural fibers, whose cross-section may be variable, and a method for making the same.
2. Description of the Prior Art:
For many years, structural elements, including those elements which must be rigid under pressure but not brittle or subject to deformation were conventionally made out of light weight metals and metal alloys, such as magnesium and aluminum. In particular, such light weight metal alloys have found application in the aerospace industry, particularly in the formation of vanes and propellers, as well as airplane and helicopter vehicle bodies, and in many transportation industries, such as the auto industry, wherein leaf springs are conventionally formed out a resilient metal such as the type mentioned. In fact, such metal articles continue to be the standard of the art today. However, these articles are expensive to produce, subject to raw material shortages and are subject to corrosion. Moreover, although constructed of light weight metals, they remain a substantial additional overall weight of the vehicle formed, thereby decreasing fuel economy.
Accordingly, there has recently been focused much attention on the production of articles and laminates including layers of "structural fibers" or stranded fibers collected into yarns. By structural fibers, it is herein meant a fiber having a substantial modulus, or yarn of the same, which is of sufficient strength to be employed in a load bearing or stress bearing capacity. In general, these fibers or yarns have a tensil modulus in excess of 5.times.10.sup.6 PSI, and include, but are not limited to such materials such as "E" glass, "C" glass, "S" glass, Kevlar.RTM. "29" and "49", carbon graphite and boron. Other similar materials will occur to those of ordinary skill in the art.
Generally, such articles are formed either by the weaving together of two or more systems of such yarns, where stress factors will allow weaving, or by non-woven stitch-bonded laminates of layers of parallel strands of structural yarns. These woven or stitch-bonded "fabrics" are subsequently saturated with a curable resin, and yields surprising strength at greatly reduced weight cost.
Such fabrics exhibit superior strength in the direction of orientation of the strands or yarns. However, one shortcoming of these fabrics is the weakness of the fabric in a direction normal to the plane of an individual layer of the fabric. This weakness is generally overcome by forming a large number of superimposed layers, either through a laborious hand setting process, or such as that disclosed in U.S. patent application No. 536,734, filed Sept. 23, 1983, now U.S. Pat. No. 4,484,459, patented Nov. 27, 1984, and co-pending herewith. Where substantial stress is to be applied in a direction normal to the plane of the fabric, or where a "vertical" (the direction normal to the working surface of the stitching or weaving machine used to form the fabric) stress is applied, or article desired, the resulting piece is extremely bulky, consists of many open spaces and includes a large number of stitching or sewing or otherwise auxillary yarns which contribute little to the overall strength of the product. When saturated and cured, the open spaces have strength given only by the resin, and are susceptible to fracture.
A further difficulty frequently experienced is the need to provide a structural article which has a complex shape, or one that varies across the transverse cross-section of the article. Thus, propeller blades and wing or edge surfaces frequently require the formation of an article exhibiting a convex curve in profile. Alternatively, many structural beams and supports require a "I" or "L" shape. However, the above-described methods for forming a structural fabric are incapable of providing such complicated shapes, other than by resort to a laborious trimming and hand lay-up process, which is neither cost effective nor capable of giving reliable products, or by first saturating and curing a fabric, and thereafter trimming it to the desired shape, which requires heavy industrial cutting equipment, and is likely to induce fracture or stress points into the article prepared.
Accordingly, a need continues to exist for the provision of a light weight structural article formed from structural fibers or stranded fibers or yarns of the same material which overcomes the above-observed shortcomings of the existing art.
Further, there is a need for a structural article comprised of structural fibers which is compact and capable of resisting stress in the direction of the fibers, as well as a direction normal thereto.
Additionally, there is a need to provide such a structural article which can easily be formed into a complex shape, or a shape which has a variable transverse cross-section.
It is the object of this invention to satisfy the above-identified needs, and further provide a structural article having unusual and desirable properties, as described below.